1. Field of the Invention
The present invention relates to a tuning fork type quartz crystal unit and a bar type quartz crystal unit, and more particularly to a tuning fork type crystal unit and a bar type crystal unit which have excellent vibration characteristics and are suitable for being reduced in size.
2. Description of the Related Art
Tuning fork type crystal units are widely used as a time reference source in wrist watches or the like. They are also used in cellular phones or the like, and there is a growing demand for reduced-size tuning fork type crystal units.
As well known in the art, three crystallographic axes X, Y, Z are defined for quartz crystals. As shown in FIG. 1, a tuning fork type crystal unit having a pair of arms 1a, 1b and base 2 is cut from a single crystal of quartz. Specifically, the tuning fork type crystal unit is cut from a Z-cut quartz plate having principal surfaces lying perpendicular to the Z-axis, such that the X-axis extends along the width of the tuning fork type crystal unit, the Y-axis along the length, and the Z-axis along the thickness. In FIG. 1, the X-axis extends horizontally, with the leftward direction being a −X direction and the rightward direction being a +X direction. Arms 1a, 1b extend in the Y direction from the respective ends of base 2 in the +X and −X directions. Each of arms 1a, 1b is in the form of a quadrangular prism, with excitation electrodes 3 disposed on respective four sides of each arm. Extension electrodes (not shown) extend from excitation electrodes 3 to base 2. In FIG. 1 and other accompanying drawings, the arrows indicated by +X, −X, Y, Z represent the directions of crystallographic axes in the quartz crystal.
On each of arms 1a, 1b, a potential is applied between a pair of excitation electrodes 3 that confront each other across the arm, and an inverse potential is applied between an adjacent pair of excitation electrodes 3. For example, if a positive potential (+) is applied between excitation electrodes 3 on the respective surfaces of arm 1a which correspond to the respective principal surfaces of the Z-cut plate and a negative potential (−) is applied between excitation electrodes 3 on the respective sides of arm 1a, then a negative potential (−) is applied between excitation electrodes 3 on the respective surfaces of the other arm 1b which correspond to the respective principal surfaces of the Z-cut plate and a positive potential (+) is applied between excitation electrodes 3 on the respective sides of arm 1b. The extension electrodes are wired to excitation electrodes 3 in order to apply those one and inverse potentials to excitation electrodes 3.
The tuning fork type crystal unit operates as follows: When the potential positive and negative potentials are applied to excitation electrodes 3 as shown in FIG. 2, an electric field is generated which is directed from the principal surfaces to side s of the Z-cut plate of arm 1a. Because of vector component P of the thus generated electric field in the +X direction, an inner side region of arm 1a is expanded in the Y direction as indicated by the single-line arrow in FIG. 3. Because of vector component Q of the generated electric field in the −X direction, an outer side region of arm 1a is contracted in the Y direction as indicated by the single-line arrow in FIG. 3. To sum up, the tuning fork type crystal unit is expanded in the Y direction when the electric field is directed from the +X direction toward the −X direction, and contracted in the Y direction when the electric field is directed from the −X direction toward the +X direction. As a result, arm 1a is tilted outward as indicated by the dual-line arrow in FIG. 3. Other arm 1b is also tilted outward as indicated by the dual-line arrow in FIG. 3 for the same reasons. Therefore, when an alternating voltage is applied to excitation electrodes 3 that are wired as described above, arms 1a, 1b cause tuning-fork vibrations at a frequency which is proportional to W/L2 where W represents the width of the arm and L the length of the arm.
However, as efforts are made to reduce the size of the tuning fork type crystal unit, the tuning fork type crystal unit suffers the problem of an increased crystal impedance (CI). Specifically, as the width of arms 1a, 1b is reduced, the area of excitation electrodes 3 is also reduced, failing to apply a sufficient amount of electric field energy to the crystal unit. The failure to apply a sufficient amount of electric field energy results in an increased crystal impedance. In attempts to solve the above problem, improved tuning fork type crystal units have been proposed as disclosed in Japanese laid-open patent publications Nos. 2002-76824 and 2002-204141 (JP, P2002-76824A and JP, 2002-204141A). According to the disclosed tuning fork type crystal units, as shown in FIG. 4, groove-like recesses 4 extending in the Y direction are defined the surfaces of arms 1a, 1b which correspond to the principal surfaces of the Z-cut plate, and excitation electrodes 3 are formed on the bottoms and sides of recesses 4. The disclosed structure is effective to increase the intensity of the applied electric field and hence to reduce the crystal impedance because the arrangement of excitation electrodes 3 makes it possible to generate a linear electric field in the X direction, i.e., along the width of the crystal unit. However, inasmuch as groove-like recesses 4 are defined in the confronting surfaces of each of arms 1a, 1b and extend longitudinally in each of arms 1a, 1b, the mechanical strength of each of arms 1a, 1b is reduced, making the tuning fork type crystal unit less resistant to shocks.